This invention relates to positive temperature characteristic (PTC) thermistor elements and PTC thermistors, and more particularly to such thermistor elements and thermistors which have a large flash resistance voltage and are adapted for use in circuits for protection against over-current, demagnetization current or motor start-up.
As shown in FIG. 13, a conventional PTC thermistor 121 may be described as having ohmic electrodes 123 and 124 formed on the two main surfaces of a planar thermistor element 122. When a voltage is applied to such a thermistor, the rush current is large at the very beginning because the thermistor 121 has a low resistance, causing it to heat up quickly and splitting it into layers across a plane approximately parallel to its main surfaces. The voltage immediately before such a laminar splitting takes place, when a rush current passes through a PTC thermistor, is called its flash resistance voltage. The flash resistance voltage tends to become small if the PTC thermistor is made smaller.